This invention relates in general to electromagnetically actuated friction clutches and in particular to an improved structure for and method of manufacturing a pole piece for use in such an electromagnetic friction clutch.
Clutches are well known devices which are commonly used in machinery to selectively connect a source of rotational power to a rotatably driven mechanism. A basic clutch structure includes an input shaft connected to the source of rotational power, an output shaft connected to the rotatably driven mechanism, and means for selectively connecting the input shaft to the output shaft for concurrent rotation. When the means for selectively connecting is engaged, the input shaft is connected to the output shaft so as to rotatably drive the mechanism. When the means for selectively connecting is disengaged, the input shaft is disconnected from the output shaft, and the mechanism is not rotatably driven. Many different types of clutches are known in the art for accomplishing this general purpose.
In some clutches, the input shaft is connected to an input member which is rotatably driven by the source of rotational power, while the output shaft is connected to an armature. The input member is fixed in a predetermined axial position. The armature, however, is axially movable relative to the input member between an engaged position, wherein it functionally engages the input member, and a disengaged position, wherein it is spaced apart from the input member. The armature is normally maintained in the disengaged position, wherein it does not frictionally engage the input member and, therefore, is not rotatably driven thereby. When moved to the engaged position, however, the armature frictionally engages the input member so as to be rotatably driven thereby. Friction clutches of this general type are well known in the art.
In some friction clutches, an electromagnet is used to cause selective movement of the armature relative to the input member between the engaged and disengaged positions. Electromagnetically actuated friction clutches of this general type operate on the principle that a magnetic field which is created about a component formed from a magnetically permeable material will exert a mechanical force on that component. This mechanical force will urge the component to move to a position of minimum resistance relative to the flow of magnetic flux (lines of force) generated by the magnetic field, usually referred to as a position of minimum reluctance. Thus, in electromagnetically actuated friction clutches, the input member and the armature are both formed from a magnetically permeable material. When the electromagnet is energized, the electromagnetic field generated thereby attracts the armature toward the input member. As a result, the armature is moved from the disengaged position to the engaged position to connect the input shaft to the output shaft, and the driven device is rotatably driven by the source of rotational power.
A typical pole piece for an electromagnetically actuated friction clutch has a generally U-shaped cross section, defining a flat pole face which is separated from the armature by a relative small air gap. Usually, the pole face of the pole piece is divided into one or more pole regions by a non-magnetically permeable material. The separate pole regions causes the magnetic flux to jump back and forth several times across the air gap separating the input member and the armature when the electromagnet is energized. As is well known, this magnetic flux discontinuity structure, or more simply flux break, increases the magnitude of the magnetic attraction between the input member and the armature. In the past, the flux break, has been accomplished by forming slots or recesses in the pole face which define relatively large air gaps between adjacent pole regions of the magnetically permeable material. Although effective to increase the magnetic attraction between the pole piece and the armature, it has been found that the formation of such slots or other recesses in pole face necessitates the removal of material from the pole piece, which weakens the strength thereof. Electromagnetic clutches of this type are often used to selectively transmit relatively high torque loads. Consequently, any weaknesses in the pole piece detract from the torque transmitting capacity of the electromagnetic clutch. Furthermore, because electromagnetic clutches of this type are often used in automotive applications, such as in air conditioner compressor assemblies, they must be relatively simple and inexpensive in construction. Accordingly, it would be desirable to provide an improved structure for a pole piece for an electromagnetic friction clutch which addresses all of these concerns.